This invention relates in general to magnetic brush developer apparatus and method for electrographic reproduction equipment, and more particularly to magnetic brush developer apparatus and method wherein a developer nap is magnetically agitated in a direction which substantially improves intermixing of the material in the nap and reduces any propensity toward toner starvation.
In the electrographic process for making reproductions of input information, an electrostatic charge pattern is formed on the surface of an insulating member in image-wise configuration corresponding to the information to be reproduced. The charge pattern is developed by applying developer material to such pattern to form a visible image. The visible developer material image is then either transferred to a receiver member and fixed to such member, or fixed to the insulating member itself. In commercial high speed electrographic reproduction equipment, a common mechanism for developing electrostatic charge patterns is a magnetic brush developer apparatus; see, for example, U.S. Pat. No. 3,703,395, issued Nov. 21, 1972 in the name of Drexler et al.
A typical magnetic brush developer apparatus includes one or more magnets located within an applicator member. The applicator member may rotate about fixed magnets, the magnets may rotate within a fixed applicator member, or both may rotate in the same or opposite directions. Developer material comprises, for example, a mixture of finely divided pigmented thermoplastic marking particles (toner) held to the surface of ferromagnetic particles (carrier) by electrostatic charges created by triboelectrification (see for example, U.S. Pat. No. 3,893,935, issued July 8, 1975 in the name of Jadwin et al). The carrier particles, with the attached toner particles, are held on the applicator member in a bristle-like formation by the magnetic fields of the magnets to form a brush nap. Alternatively, the developer material is of the type comprised solely of charged marking particles which also exhibit magnetic properties, referred to as single component developer (see for example, U.S. Pat. No. 4,171,274, issued Oct. 16, 1979, in the name of Schwarz et al). The developer material brush nap is moved into contact with the electrostatic charge pattern by brushing the nap bristles across the surface of the insulating member. When the developer material contacts the surface of the insulating member, the electrostatic attraction for the charged toner particles by the charge pattern on the insulating member overcomes the attraction of the carrier particles for the toner particles (or the magnetic attraction of the magnets within the applicator member for single component developer). Thus, the toner particles are transferred to the charge pattern and the pattern is developed.
Since the developer material is presented to the charge pattern only in the area of contact of the nap bristles and the insulating member, there is a limited amount of material in the bristles available at any given instant to carry out development. In most instances sufficient developer material is available for complete charge pattern development. However, when the insulating member is moving at a relatively high speed relative to nap bristles, or when the insulating member has a large area of dense charge patterns, there may be insufficient developer material (toner starvation) in that segment of the bristles contacting that portion of the charge pattern for complete development. This results in defects in the developed image such as washed out areas, or white streaks or spots in the reproduction.
One arrangement for increasing the developer material available for charge pattern development is shown in the Research Disclosure Bulletin of August 1982 at p. 299 (No. 22010). In this arrangement, the nap bristles on an applicator shell are agitated throughout the development zone by counter rotating magnets adjacent to the development zone. Such agitation causes the bristles to rapidly tumble or flip presenting additional developer material to the charge pattern for development. Another arrangement is shown in copending U.S. patent application Ser. No. 474,901 filed Mar. 14, 1983 in the name of W. C. Lu. In this latter arrangement, only a central portion of the nap bristles in the development zone is agitated, with the non-agitated portion forming a curtain about such central portion. Thus, the agitated developer material is sealed from the environs by the non-agitated portion of the nap bristles. While these described arrangements markedly increase the available developer material for effecting charge pattern development, agitation occurs only in the direction of travel of the brush nap on the applicator. This can lead to starvation of developer material of the nap in areas of solid pattern development, while developer material in adjacent transversely spaced areas remain plentiful.